Solid–liquid Phase Equilibria and Volumetric Properties of Alkali Borates at 308.2, 323.2, and 348.2 K for Boron-Rich Brine

The Zabuye Salt Lake of the Tibet Plateau in China is rich in lithium and boron resources. To optimize the comprehensive utilization of salt lake brine, the solid–liquid phase equilibria of the ternary system Li2B4O7–Na2B4O7–H2O were determined at 308.2, 323.2, and 348.2 K using isothermal dissolution equilibrium method. The phase diagrams of the ternary system Li2B4O7–Na2B4O7–H2O all contained one invariant point, two solubility curves, and two solid-phase crystallization regions without any solid solutions or complex salts. At 308.2 and 323.2 K, the corresponding equilibrium solid phases were Li2B4O7·3H2O and Na2B4O7·10H2O, while at 348.2 K, they were Li2B4O7·3H2O and Na2B4O7·5H2O. The solution density increases with higher Li2B4O7 and Na2B4O7 content, peaking at the invariant point. The unreported Pitzer model parameters (βMX(0), βMX(1), CMXϕ, θLi+,Na+, ΨLi+,Na+,B4O5(OH)42–) and solubility products Ksp were obtained to model the phase equilibria. The unreported volume parameters (β(0),V, β(1),V, CV, θLi+,Na+V, ΨLi+,Na+,B4O5(OH)42–V) were obtained to further study the thermodynamic volume properties in borate solution, and the related density, excess molar volume, and apparent molar volume were modeled in detail. This work is conducive to the comprehensive utilization of boron-rich salt lakes.